This invention relates to a method for the production of epitaxial films of single crystalline characteristic and, in particular, to a method for depositing doped II-VI epitaxial layers, such as, ZnSe, by organometallic chemical vapor deposition (OM-CVD). Epitaxial films prepared in accordance with this invention may be prepared from volatile compounds and elements of zinc, cadmium and mercury with volatile compounds and elements of sulfur, selenium and tellurium. Compounds to consider are the binary and ternary compounds within these groups of elements.
There is voluminous prior art involving the production of II-VI epitaxial films and devices and, in particular, zinc selenide (ZnSe) films and devices. ZnSe is a direct bandgap semiconductor with a bandgap of 2.67 eV at room temperature. Because of its wide bandgap, ZnSe has a high potential for light emitting devices emitting radiation in the visible range of the spectrum and this is why there has been so much interest in this semiconductor compound in the past. To date, however, there has been no long term success in the employment of ZnSe and its II-VI companions in semiconductor devices.
Examples of prior art are found in U.S. Pat. Nos. 3,173,814; 3,312,571 and 3,364,084. Examples in the literature are the articles, "The Luminescence Center in Self-Activated ZnS Phosphors, J. S. Prener and D. J. Weil, Journal Of The Electrochemical Society. Volume 106, page 405 (1959). "The Use of Metal-Organics in the Preparation of Semiconductor Materials", H. M. Monasevit and W. I. Simpson, Journal of Electrochemical Society, Volume 118, No. 4, p. 644 et al (1971); "Growth & Characterization of Undoped ZnSe Epitaxial Layers Obtained By Organometallic Chemical Vapor Deposition", P. Blanconnier et al, Thin Solid Films, Volume 55, pp 375-886 (1978); "Organometallic Vapor Deposition of Epitaxial ZnSe Films on GaAs Substrates", Wolfgang Stutius, Applied Physics Letters, Volume 33(7), Oct. 1, 1978; "Luminescence in Highly Conductive n-Type ZnSe", J. C. Bouley et al, Journal of Applied Physics, Volume 46(8), August 1975; "High Conductivity ZnSe Films", Julio Aronovich et al, Journal of Applied Physics, Volume 49(4), page 2584 et al, April, 1978. Also, there is the book of "Physics & Chemistry of II-VI Compounds", edited by M. Aven and J. S. Prener and published and distributed by American Elsevier Publishing Co., Inc. (1967) which discusses, for example ZnSe beginning on page 596.
Early work on ZnSe focused on the properties of bulk crystals which are grown by either vapor transport or by direct reaction between the elements Zn and Se at high temperatures and high pressures. The crystals are usually rather small and contain a large number of intrinsic defects (dislocations, vacancies, etc.) due to the high growth temperature. The as-grown, highly resistive bulk ZnSe crystals (.rho.&gt;10.sup.12 .OMEGA..cm) can be converted to low resistivity n-type material by diffusing a group III element (Al, Ga, or In) at high temperatures, followed by an anneal in zinc vapor. This "zinc extraction treatment" reduces the number of zinc vacancies which form complexes with the donor atoms and act as deep trapping centers. The doped crystals obtained by these processes are, however, still highly compensated and contain a large concentration of unwanted deep acceptor states. This is usually evident in the large intensity of the self-activated (SA) part of the photoluminescence spectrum. Also, carrier concentrations and mobilities are low compared to Ge, Si, or the III-V compound semiconductors.
Although studies of bulk ZnSe crystals are useful to clarify some of the fundamental aspects, the preparation of high quality thin films is certainly desirable from device point of view. Since single crystalline ZnSe substrates with a low dislocation density are difficult to obtain, most of the thin film studies are done on GaAs or Ge substrates. ZnSe is lattice-matched rather well to GaAs and Ge, with a .about.5.667 .ANG. for ZnSe, a .about.5.6534 .ANG. for GeAs, and a .about.5.658 .ANG. for Ge at room temperature. Thin epitaxial films of ZnSe have been prepared by vapor transport evaporation, sputtering, liquid phase epitaxy [e.g. the article, "Shallow Acceptors and P-type ZnSe", [K, Kosai et al, Applied Physics Letters, Volume 35, p. 194 (1979)], molecular beam epitaxy [T. Yao et al, Applied Physics Letters, Volume 35, p. 97 (1979)], and organometallic chemical vapor deposition (OM-CVD) [e.g., the articles of P. Blanconnier et al and Stutius, supra]. The experimental results suggest that the material obtained by the last two methods is superior in quality, as judged by the excellent surface morphology and the low concentration of deep centers seen in photoluminescence studies.
The preparation of thin layers of ZnSe by OM-CVD is especially appealing because of its simplicity and versatility. This deposition method can also easily be adapted for the fabrication of inexpensive large area devices. In the OM-CVD process, ZnSe is formed by reacting dimethylzinc or diethylzinc with hydrogen selenide inside a quartz reactor. Details of the reactor design and the growth conditions have been published [e.g., the Article of Stutius, supra]. The remaining small lattice mismatch between the GaAs or Ge substrates and the ZnSe layers can be overcome by adding a small amount of H.sub.2 S to the gas stream during the deposition to form ZnS.sub.x Se.sub.1-x. The exact lattice match to GaAs occurs for x=0.052 and to Ge for x=0.035. The results from photoluminescence studies suggest that the interface between ZnS.sub.x Se.sub.1-x and the GaAs substrates is much improved near the lattice-matched composition. See the article, "Photoluminescence and Heterojunction Properties of ZnS.sub.x Se.sub.1-x Epitaxial layers on GaAs and Ge Grown By Organometallic CVD", Wolfgang Stutius, Journal of Electronic Materials, Vol. 10(1), January, 1981. This is evident from the width of the near-bandgap PL peak and the intensity of the self-activated PL which both have a mimimum near x=0.05 signifying a relief of the strain and a reduced number of defects at the interface.
ZnSe and ZnS.sub.x Se.sub.1-x layers prepared by OM-CVD without intentional doping are highly resistive. From C-V measurements of ZnSe layers deposited on n-GaAs and from vander-Pauw resistivity and Hall effect measurements of ZnSe layers deposited on Cr-doped seminsulating GaAs substrates we concluded that the electrical resistivity of these layers is greater than 10.sup.5 .OMEGA..cm.
The direct synthesis of highly conductive ZnSe is of great technological interest because the availability of low resistivity n and p-type ZnSe will lead to the successful fabrication of II-VI electroluminescent devices which is yet to be a commerical reality in face of all the past work in this area of technology.